Revamps 2026 Issue

was tightened up to ensure steam flow responded more dynamically to ambient tem- perature changes rather than operating on a fixed setpoint. Better integration with the overall process control sys- tem meant that steam con- sumption tracked actual need rather than running at a con- servative fixed rate regard - less of conditions. The result was a meaningful reduction in steam usage without com- promising the thermal pro- tection of the tubes. Case 2: Strategies to

Bae

Vibrating tube (amplitude exaggerated)

B 3

Shell-side cross-flow zigzag baffle path

Figure 2 Baffle geometry matters more than it sometimes gets credit for

mitigate vibration in high-pressure heat exchangers Among all heat exchangers in a refinery, high-pressure exchangers occupy a special category. The pressures are high, often exceeding 125 kg/cm²g. Temperatures hover around 450°C, especially in reactor feed/effluent service. The streams are hydrogen-rich, frequently two-phase, and in many cases carry fouling tendencies that make thermal performance difficult to sustain over time. These are not for - giving services, and the consequences of getting something wrong, such as a tube leak, baffle failure, or tube fracture, go well beyond a maintenance call. In hydrogen service at high pressure, a tube failure is a process safety event. What makes these exchangers particularly challeng - ing from a mechanical standpoint is the combination of demanding operating conditions and the fact that they are almost always running in a configuration where multiple shells are arranged in series. Whether for flow maldistri - bution between shells, slug flow in two-phase regimes, or pressure pulsations at the inlet, these can interact in ways that are difficult to predict at the design stage and to diag - nose once the unit is running. Case background The pattern in this type of service tends to follow a recog - nisable sequence. Vibration begins as shell-side inlet flow is increased, typically during a push for higher throughput or recovery from a process upset. It may begin subtly, as a low-frequency oscillation that operators notice but can - not immediately attribute to a specific cause. Then the flow fluctuations become more pronounced. Pressure drop across the exchanger starts behaving erratically. At some point, the decision is made to bypass reactor feed around the exchanger to stabilise the loop, which immediately reduces heat recovery and forces the down- stream heater to pick up the slack through increased fuel firing. Energy costs go up, the high-pressure loop becomes harder to control, and the maintenance team starts watch - ing the exchanger closely for signs of tube leakage. This is not a hypothetical sequence; it plays out with some regu - larity in reactor feed/effluent services, and the underlying driver is almost always flow-induced vibration (FIV).

Several contributing factors typically work together to create the problem. High local cross-flow velocities on the shell side are often the starting point, particularly in two- phase flow regimes where the gas and liquid phases are not evenly distributed across the bundle cross-section. When shells are arranged in series, slug flow developing in one shell can propagate and amplify through the down- stream shells, turning what might be a manageable local fluctuation into a system-wide instability. Piping configura - tion upstream of the exchanger inlet frequently plays a role as well: where a bypass line rejoins the main process line and the mixing point is too close to the exchanger nozzle, the resulting flow non-uniformity enters the shell before it has any chance to settle. Industry licensor standards often specify minimum straight lengths of 10D to 20D after such mixing points for exactly this reason. However, these requirements do not always survive the pressure of plot space constraints dur- ing detailed engineering. Moreover, the geometry of baffles is often underestimated in its importance. Horizontal-cut baffles in two-phase service create conditions where liquid ‘pools’ at the bottom of the shell, while vapour preferen - tially channels through the upper section. This generates pressure pulsations and uneven tube loading that directly contribute to vibration (see Figure 2 ). When all of these factors are present simultaneously, the exchanger is essentially being tasked to handle a service it was never quite designed to manage reliably. Troubleshooting measures The starting point for any vibration investigation in high-pressure exchangers is a careful comparison of the design temperature and flow profiles with what the unit is actually seeing in operation. Process conditions in reactor feed/effluent service tend to drift over time. Feed compo - sition changes, reactor conversion varies with catalyst age, recycle gas purity shifts, and what was a well-behaved single-phase or mild two-phase service at commissioning may have evolved into something significantly more chal - lenging years later. Mapping local velocity profiles along the exchanger length, not just at the nozzles, often reveals

Revamps 2026